My Theory:
Fenrir is born to kill Odin. He killed Odin during Ragnarok.
Cerberus is the guardian of the world of deaths in Ancien Greece. He's not evil. He stops alive people to enter and dead people to go out. He has a job and does it seriously.
Does it mean that Naberius Narnia(who took the name Fenrir during the Deviculum) will kill someone important? Hummm.
